At 01:28 PM 4/23/2004 -0400, you wrote:


>OK so let's rethink the problem. I think my posting on using an opamp made
>it to the list. It should be possible to configure an opamp as a inverting
>comparator that's close to rail to rail. Let's start with the opamp mantra:
>
>The output tries it very best to equalize the voltages on the + and - inputs.
>
>OK. Since we want to invert the output we need a situation where the output
>will drive low when an input is high and vice-versa. We use the game plan
>of putting one input at 2.5V using 47-100k voltage divider and using the
>PIC output to swing the other terminal. In fact if we wanted to we could
>use a dual opamp like a 1458 and drive the two LED terminals, one positive
>and one negative. But back to the issue.
>
>To drive the opamp output low the + terminal needs to negative in
>reference to the - terminal. So that means we can either ground the
>+ terminal and have the - @ 2.5V or have the + terminal @ 2.5V and make
>the - terminal high (+5V).
>
>To drive the opamp output high the + terminal needs to positive in
>reference to the - terminal. So that means we can either ground the
>- terminal and have the + @ 2.5V or have the - terminal @ 2.5V and make
>the + terminal high (+5V).
>
>One last thing is that the PIC output needs to be the opposite polarity of
>the opamp output. So in the first instance the PIC output is high and in
>the second the PIC output is low.
>
>So we have a winner. The + terminal needs to be 2.5V with the divider and the
>PIC output connected to the - terminal. That way the opamp drives low when
>the PIC output is high and vice-versa. A inverting comparator.
>
>Hope this helps, 2 resistors and a 8 pin package opamp/comparator will do the
>trick.

Sure, plus a series resistor for the LED. Note that some op-amps don't
play well when you use them as comparators (they may have diodes from V+ to V-,
for example). Comparators are often open-collector. Many op-amps will work
fine.

>BAJ

Or my posting to the EE thread, where we have this 1.5 component solution:


                                     LED A
                                 +---|<--+-------+
                                 |       |       |
                                 +--->|--+      .-.
                                 |   LED B      | | Rs
                                 |              | |
                                 |              '-'
                                 |   |\          |
                   PIC  ---------+---| >O--------+
                                     |/

                          1/2 SN74LVC2G14


PIC pin:        High-Z: both LEDs off
                 Low     LED A on
                 High    LED B on


The SN74LVC2G14 dual ST inverter is about 0.48 one-off, 0.27 in 100's,
and is housed in a compact SOT-23-6, so it won't break the bank.

By floating the PIC port pin, both LEDs can be turned off, with
essentially zero current drain (thanks the the ST input).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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